The present invention relates to multiple-beam longitudinal-interaction electron tubes such as for example klystrons or travelling-wave tubes. These tubes which are built around a main axis comprise several longitudinal electron beams parallel to this main axis. These beams are generally produced by a common electron gun provided with several cathodes. They are collected at the end of travel in one or more collectors. Between the gun and the collector, they cross a body which is a microwave structure at whose output microwave energy is extracted. This structure may be formed by a sequence of resonant cavities in the case of a klystron or a microstrip line in the case of a travelling-wave tube. The electron beams, in order to keep their long and thin shape, are focused by a focusing device that is centered on the main axis and surrounds the microwave structure.
The advantage of multibeam electron tubes as compared with single-beam tubes is that the current produced is higher and so is the power, or else the high voltage and the length are lower.
The space requirement of the tube for equal current values is considerably smaller. The electrical supply and the modulator used are thus simplified and more compact.
The insulation in the gun can be obtained in air whereas in a single-beam tube with equivalent current, it is necessary to use oil or sulfur fluoride or any other insulating medium.
The interaction yield is improved owing to the generally lower perveance of each of the beams.
The passband of the multibeam klystrons is widened because the cavities are charged with higher current than in the single-beam configuration.
As compared with single-beam tubes, the major drawback is that it is difficult to generate an optimum focusing magnetic field. This is due especially to the fact that there is an absence of symmetry of revolution between the focusing device and each of the beams. The axial magnetic field produced by the focusing device is not axisymmetrical with respect to the axis of each of the beams. In a single-beam tube, the axis of the focusing device is merged with that of the electron beam and the axial magnetic field that it produces has a symmetry of revolution around the axis of the beam.
Another reason is that it is difficult to make a gun so that it will produce appropriate electron beams. The electron beams must be as close as possible to the main axis of the tube in order to reduce the defocusing radial magnetic fields which increase with distance from the main axis. However, the closer we come to this axis the smaller is the amount of space available. The cathodes therefore need to be very close to one another and must have a small surface area.
In the case of the klystrons, the distance between two neighboring beams is dictated by the geometry of the cavities, the diameter of the drift tubes between two cavities and the mode in the cavity.
The fact of seeking to bring the electron beams together makes it necessary for the cathodes to have a small emissive surface and a very great current density, thus considerably reducing their lifetime. Compromises between all these constraints have to be obtained.
To enable an increase in the distance between the cathodes and a reduction of their current density without placing the beams at a distance from the main axis, it has been proposed to position the cathodes on the concave part of a generally spherical cap. Their current density may be reduced and the electron beams may converge towards the body of the tube.
A ring-shaped pole piece generally surrounds the gun at the level of all the cathodes. Locally, the axial magnetic field is not symmetrical with the axis of each of the beams and the beams undergo a deflection and may be intercepted by the walls of the drift tubes and of the cavities. This arrangement is appropriate only for low-convergent cathodes.
The present invention seeks to optimize the magnetic field of a multibeam electron tube, especially in the vicinity of its cathodes so that the risks of interception are reduced.